This disclosure relates to polymeric compositions, methods of manufacture thereof and to articles comprising the same.
Polymeric compositions, especially elastomers and rubbers, are cured by vulcanization and/or other physical crosslinking processes to produce materials that can be deformed to great extensions and recover elastically. When an elastomer recovers its original dimensions it is described as having returned to a state of zero strain or (equilibrium state). A number of properties, including stiffness (modulus), degree of swelling, coefficient of thermal expansion, of elastomers are all strongly affected by the degree of crosslinking produced in the material.
It is well known that the amount of true stress (force per unit instantaneous area, σt) required to stretch an ideal elastomer to an extension λ (final length divided by the undeformed length) is calculated from equation (1) below:
                              σ          t                =                              G            r                    ⁡                      [                                          λ                2                            -                              1                λ                                      ]                                              (        1        )            where Gr is the shear modulus. It is also known that the shear modulus of an ideal elastomer or rubber is related its crosslink density by the relationship expressed in equation (2):
                              G          r                =                              ρ            ⁢                                                  ⁢            RT                                M            c                                              (        2        )            
The elasticity of an elastomer is due to entropic changes in the molecules when the elastomer is subjected to stress. These entropic changes result in a transfer of heat between the elastomer and its surroundings. For example when an elastomer is stretched (e.g., loaded) heat is transferred from the elastomer to its surroundings. Conversely, when the elastomer is unloaded, heat is absorbed by the elastomer from its surroundings. The transfer of heat between the elastomer and its surroundings occurs by diffusion, which can take time because of the relatively low thermal conductivity of these materials. This results in hysteretic heating during cyclic loading. Hysteretic heating has a number of detrimental effects that can include premature failure in cyclic applications, increased permanent set (i.e., loss of recoverable elasticity), and energy loss through dissipation. These detrimental properties of elastomers lead to increased material costs and to increased maintenance costs when elastomers are used in commercial applications where they are subjected to cyclic loading.
Extensive research and development has therefore been directed toward improving the properties of elastomers. For example, a reduced coefficient of thermal expansion and degree of swelling can be obtained by increasing the crosslink density of the elastomer, but this comes at a cost of increased stiffness (modulus) and a corresponding reduction in elongation to break and potentially strength. Fillers like carbon black and other particles have been used to overcome some of these tradeoffs, but they have all shown to increase the hysteresis of the material and make the rubber more difficult to process.
It is therefore desirable to develop polymeric compositions with a unique balance of properties between tensile strength, toughness, impact resistance, tear strength, flex resistance, reduced hysteresis, fatigue, longer service life and resistance to swelling amongst other properties. For example, many seal applications could us a low modulus (soft) elastomer that has a very low coefficient of thermal expansion and a very low degree of swelling. With conventional curing and processing techniques, simultaneously achieving all of these properties is not possible.